Have you been able to inspect the fish and perhaps notice sores or even a blueish film on them? I've witnessed bluegill only die-offs around my parts during the early spring (which is later than yours!). Sometimes, bacterial or protozoal infections can just plague one species. You will usually see external signs like I mentioned, but not always. I see this in the early spring and think it might be related to continual fluctuations with water temps. This can induce stress which makes infections more liekly.
